1. Field of the Invention
The present invention relates to a beam compression process for an antenna pattern which eliminates distortions of a received pattern produced during a process of compressing an antenna pattern of antennas, to thereby improve beam compression performance.
Generally, a beam width is one of indices of quality of antenna patterns of receiving antennas and so forth. The smaller the beam width, the higher the performance of the antenna pattern. However, the beam width and the size (length) of an antenna are inversely proportional to each other. Thus, a reduction in the beam width increases the antenna size, while a reduction in the antenna size increases the beam width.
In an attempt to double the power of discriminating an object, i.e., resolution, in a radar antenna, for example, the beam width must be halved, typically in doubling of the antenna size. The doubled antenna size has various drawbacks, including an increase in not only the area occupied by the antenna, but also the weight of the antenna and dimensions of the antenna support structure. Conversely, if the antenna size is halved, the beam width is doubled and the discriminating power deteriorates to half of the original level.
It is well known that the beam width and the antenna size are contradictory to each other, as mentioned above. Since actual antennas are subjected to limitations in the area occupied by the antenna and other factors in most cases, practical compromises must be made to some extent in limiting beam width under such limitations.
For the purpose of improving the above problem, there has been conventionally known a beam compressing process using the principle of a multiplicative array in which the beam width is reduced by multiplying received signals of a plurality of antennas by each other. FIG. 1 is a diagram showing an antenna arrangement for carrying out such beam compression. Denoted by reference numeral 101 is a main antenna constituted by, for example, an array antenna comprising a plurality of radiation elements arrayed into the rectilinear form with equal intervals, and 102 is a sub antenna. The sub antenna 102 is arranged at a position spaced from the main antenna 101 in the X direction, i.e., the direction where a beam width is to be compressed. 103 is a multiplying circuit for multiplying a received signal of the main antenna 101 by a received signal of the sub antenna 102. In the antenna device thus arranged, the signals received by the antennas 101, 102 are input in phase to the multiplying circuit 103 and subjected to a multiplication process. As a result, a directional characteristic of the main antenna and a directional characteristic of the sub antenna are multiplied to give a synthetic directional characteristic with the beam width compressed therein.
However, the above-explained conventional beam compression process for an antenna pattern has the problem that although an identical effect to that resulted from beam compression can be obtained, a grating lobe of the sub antenna forms a received pattern indicative of a radio wave source is present, in spite of the absence of any radio wave source, i.e., thereby causing distortions in the received pattern.